1. Field of the Invention
The present invention relates to photographic optical systems, and more particularly relates to a photographic optical system suitable for optical apparatuses, such as digital still cameras, video cameras, and silver-film cameras.
2. Description of the Related Art
Hitherto, in optical apparatuses, such as silver-film cameras, video cameras, and digital still cameras, a micro lens or a macro lens (hereinafter both referred to as a xe2x80x9cmacro lensxe2x80x9d) has been known as a photographic lens primarily for photographing close-up objects.
The depth of field of a macro lens is generally much less in a high-magnification photographing mode (that is, a close-up photographing mode). This makes it difficult to correct the focus using a focusing lens unit during high-magnification photography, which easily causes the imaging relationship between the optical system (the photographic lens) and an object to deviate from the in-focus state along the optical axis direction. In general, the displacement of the focal point due to the deviation in the imaging relationship between the optical system and the object from the in-focus state is corrected by a configuration which employs an auto-focus system to detect the amount of defocusing and to move the focusing lens unit so as to make fine adjustments. However, since the macro lens, which is primarily used for close-up photography, has very little depth of field, even a slight deviation of the imaging relationship between the optical system and the object during exposure on a light-sensitive surface (during photographing) may cause the image to become out of focus by more than a tolerable amount. Focusing is therefore required whenever the imaging relationship is deviated, even during exposure. In order to obtain a high photographic magnification, the macro lens generally causes a plurality of focusing lens units to move (floating) or causes a heavy focusing lens unit to move. This deteriorates the tracking ability of the focus, making it difficult to correct the defocus during exposure.
In addition, during the high-magnification photography, blurring of the photographic image plane due to camera shaking or the like becomes noticeable. In Japanese Patent Laid-Open No. 07-294853 (corresponding to U.S. Pat. No. 5,598,299), and Japanese Patent Laid-Open No. 08-201691 (corresponding to U.S. Pat. No. 5,751,485), in order to correct blurring of the photographic image plane during close-up photography (a photographic magnification of 0.5 to 1.0), there have been proposals to limit the reduced image quality due to blurring of the photographic image plane by moving some of the lens units in the direction perpendicular to the optical axis.
In Japanese Patent Laid-Open No. 08-101361 (corresponding to U.S. Pat. No. 5,790,309), there is disclosed an optical system that has a vibration-canceling unit (vibration-canceling lens unit), which is moved in the direction perpendicular to the optical axis in order to correct blurring of the photographic image plane due to decentering of the optical axis, and that moves the focusing part in accordance with the moving of the vibration-canceling unit in order to correct the displacement of the focal point during vibration canceling.
Since the depth of field of the macro lens is generally small in the high-magnification photographing mode, it is difficult to correct the focal point. Thus, there is a problem in that even a slight displacement of the object distance, particularly during photographing, causes defocus.
Furthermore, blurring of the photographic image plane due to slight camera-shaking decreases the optical performance.
It is an object of the present invention to provide a photographic lens that can rapidly correct the displacement in the focal point even though the imaging relationship deviates during photography (during exposure on the light-sensitive surface) and that can always provide superior optical performance even in, for example, a close-up photographing mode.
To this end, according to a first aspect of the present invention, there is provided a photographic optical system for forming an image of an object on a light-sensitive surface, including a first focusing part for focusing while exposure is not performed on the light-sensitive surface and a second focusing part for focusing while exposure is performed on the light-sensitive surface.
For example, a photographic optical system includes, in order from the object side to the image side thereof, a first lens unit having positive optical power, a second lens unit having negative optical power, a third lens unit having positive optical power, and a fourth lens unit having positive optical power, wherein the second lens unit moves along an optical axis toward the image, and the third lens unit moves along the optical axis toward the object while focusing from an infinity distant object to a closest object, and the fourth lens unit moves along the optical axis so as to correct the displacement in focal position while exposure is performed on the light-sensitive surface.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.